System and method for displaying the location of a ferromagnetic object in a living organism

ABSTRACT

Described is a system and/or method for displaying the location of a ferromagnetic object in a living organism by using a surgical probe. The surgical probe has a shaft with three-dimensional magnetoresistance sensors located on a distal end configured for insertion into the living organism and three-dimensional magnetoresistance sensors located on a proximal end that stays outside of the living organism. The system comprises a display configured to show the relative location of a detected ferromagnetic object to the tip of the probe in a simulated three-dimensional view on a two-dimensional display.

This application is a continuation-in-part of U.S. application Ser. No.16/947,890 filed 23 Aug. 2020, the entire contents of which areincorporated by reference herein.

FIELD OF INVENTION

The present invention relates to systems and methods for detecting anddisplaying unintended foreign objects inside of a human or other animal.Embodiments of the invention can assist human or veterinary surgerypersonnel to detect and retrieve retained ferromagnetic or ferrimagneticforeign bodies during or after surgery. Examples of foreign bodies thatcould be detected and removed can include tools, surgical needles, andsponges that comprise ferromagnetic or ferrimagnetic material.

BACKGROUND

Retained foreign bodies occur in 0.03% to 0.1% of human abdominalsurgical cases. (Source: US National Library of Medicine). Foreignobjects can inadvertently be retained as a result of: (a) a miscount bythe operating room staff; (b) a change in surgical technique from thestandard protocol/steps; (c) defective surgical materials, instrumentsor equipment; (d) a prolonged surgical case with multiple operating roomteams; (e) and/or when multiple procedures are performed synchronouslyor successively under the same anesthetic. When an unintended retainedforeign body was present after surgery, about 30% of patients requiredreadmission and 83% of patients required reoperation. The average costof an unintended retained foreign body after surgery was about $95,000in 2008, including about $15,000 in medical costs with the remainderbeing for legal expenses (Source: journals.sagepub.com).

The prior art for minimizing retained bodies includes optimizedcommunication by

operative team members, surgical counts, bar code scanning,radio-frequency detection and identification, and x-ray technology.Problems with manual and team methods includes miscounts reported ascorrect, surgeon error, and defective surgical materials or equipment.

Prior art radio-frequency based systems are typically limited to itemswith radiofrequency tags. Such radio-frequency approaches are notdesigned to identify small items such as retained needles nor do theseprior art techniques easily localize tagged items.

X-ray/fluoroscopy detection has many limitations. The efficacy of x-rayis limited based on patient body habitus (x-ray penetration), surgicalposition (affects how x-ray can be delivered and penetrate the patient),and accessibility for radiologist reading during surgery, as well assize of retained item for an accurate reading. There is also an exposurerisk to the patient and operating room team from the radiation,especially during pregnancy, etc.

Limited access and visualization of the surgical field complicatesvisual detection of lost, dislodged, or retained foreign bodies duringminimally invasive surgery. Foreign objects or fragments can be unseenand/or lost in the surgical field. A camera and handheld or roboticinstruments to manipulate the operative field are typically used in such“keyhole” procedures.

Metal detectors that rely on electrical conductivity have difficultyfinding small retained objects (needles, suture fragments, staples,etc.). Such metal detectors generate an electromagnetic field, using analternating current in a conductive coil, to produce eddy currents inconductive metal and then sense changes to the electromagnetic fieldcreated by the induced eddy currents. Small metal fragments, and smallitems such as surgical needles, have a small cross-sectional area, andtherefore the induced eddy currents do not produce a detectable changeto the electromagnetic field. It is also preferable to have a foreignobject detector that does not generate electromagnetic field, which someconsider unhealthy.

If a foreign body is detected, it needs to be located inthree-dimensional space. Therefore, it is highly desirable to have adisplay that can clearly and accurately guide the operating roompersonnel to the location of the foreign body.

If a presumed foreign body cannot be located or localized usingavailable detection methods, the surgical procedure must be convertedfrom a minimally invasive approach to traditional open surgery. In theevent of conversion to open conventional surgery, length of patienthospitalization, pain medication requirements, time to return to regularactivity, overall patient morbidity, and cosmesis requirements allincrease.

It is also desired to have better systems and methods for detectingferromagnetic objects in situations other than surgery. One example isthe need to non-invasively detect ferromagnetic objects in a human bodyprior to conducting magnetic resonance imaging (MRI).

For the above reasons, better tools and techniques are needed fordetecting a retained foreign body. A safe, effective, and low-costsystem and method for detecting items used during conventional andminimally-invasive surgery would save many lives, reduce surgical costs,and reduce legal malpractice costs.

BRIEF DESCRIPTION OF THE DRAWINGS

This disclosure is described in conjunction with the appended figures inwhich:

FIG. 1A shows a surgical ferromagnetic object detector;

FIG. 1B shows an alternate embodiment of a surgical ferromagnetic objectdetector;

FIG. 1C shows the ferromagnetic object detector of FIG. 1A used in apatient;

FIG. 2 shows the detector of FIG. 1A with the probe tip inserted into abody cavity;

FIG. 3 shows a two-dimensional view of the influence of a ferromagneticobject on (a) magnetic flux lines and (b) a pair of magnetic sensors inthe probe of FIG. 1A;

FIG. 4A is a tunneling magnetoresistance (TMR) Wheatstone bridge circuitfor use in the ferromagnetic object detectors of FIG. 1A to FIG. 3 ;

FIG. 4B shows the voltage response of the TMR circuit of FIG. 4A tosmall changes in magnetic field strength;

FIG. 4C shows the voltage response of the TMR circuit of FIG. 4A tolarge changes in magnetic field strength that saturate the TMR sensor;

FIG. 4D shows two ferromagnetic layers separated by an insulatingbarrier of a magnetic tunnel junction in a TMR sensor;

FIG. 5 shows a 3-axis sensor unit that comprises three TMR circuits ofFIG. 4A;

FIG. 6 shows a-3-axis TMR sensor module similar to the sensor unit ofFIG. 5 ;

FIG. 7 shows two integrated 3-axis TMR sensor modules configured for useas part of the ferromagnetic object detectors FIG. 1A to FIG. 3 ;

FIG. 8 is a block diagram of the ferromagnetic object detectors of FIG.1A to FIG. 3 ;

FIG. 9 shows a method for calibrating the system of FIG. 1A to FIG. 8 ;

FIG. 10 shows information displayed by the system of FIG. 1A to FIG. 8 ;

FIG. 11A to FIG. 11F show the 3-dimensional display of FIG. 10 fordifferent positional relationships between the probe tip andferromagnetic object;

FIG. 12 shows a configuration of an alternate magnetic probe that hasmore than two magnetic sensors and can be used for pre-MRI detection offerromagnetic objects;

FIG. 13A shows a medical ferromagnetic object detector having a singlemagnetoresistance module at the probe tip and at the probe base;

FIG. 13B shows ferromagnetic object detector having a plurality ofmagnetoresistance modules at the probe tip and two magnetoresistancemodules at the base;

FIG. 14A shows details of region A-A and region B-B of FIG. 13B;

FIG. 14B shows a two-dimensional example of how the relative distanceinformation from the sensor output of the embodiment shown in FIG. 13Band FIG. 14A can be used to calculation the location of a ferromagneticobject;

FIG. 15A shows a ferromagnetic object detector having athree-dimensional array of magnetoresistance modules at the probe tipand at the probe base;

FIG. 15B and FIG. 15C show enlarged views of the probe tip sensing arrayand the probe base sensing array of FIG. 15A; and

FIG. 16A and FIG. 16B show a ferromagnetic object detector withmagnetoresistance modules distributed along a two-dimensional plane ofthe probe shaft and the use of zone-based sensing.

It should be understood that the drawings are not necessarily to scale.In certain instances, details that are not necessary for anunderstanding of the invention or that render other details difficult toperceive may have been omitted. It should be understood that theinvention is not necessarily limited to the particular embodimentsillustrated herein.

DETAILED DESCRIPTION

The ensuing description provides preferred exemplary embodiment(s) only,and is not intended to limit the scope, applicability or configurationof the disclosure. Rather, the ensuing description of the preferredexemplary embodiment(s) provides those skilled in the art with anenabling description for implementing a preferred exemplary embodiment.It should be understood that changes could be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

Specific details are given in the following description to provide athorough understanding of the embodiments. However, it will beunderstood by one of ordinary skill in the art that the embodiments maybe practiced without these specific details. For example, those skilledin the art will know that different circuitry, transducers, materials,processes, configurations, and components may be substituted.

1. Definitions

In embodiments of the invention and claims, magnetism is physicalphenomenon that works similarly to the attraction for pure iron produceby lodestone. A magnetic source is any material or process (e.g.electromagnetism) that creates magnetism. A ferromagnetic material is asubstance that exhibits magnetism because (a) it behaves like lodestonein being attracted to pure iron (i.e. it is a permanent magnet) or (b)it behaves like pure iron in being attracted to a permanent magnet.Examples of ferromagnetic materials include, but are not limited to,cobalt, iron, various ferric oxides, nickel, and rare earth magnets.

In embodiments of the invention and claims, a magnetic field is a regionaround a magnetic source in which a magnetism acts. Magnetic fields canoccupy a large region (e.g. earth's magnetic field) or they can bedetectable in only a small region, such as the electro-magnetic fieldsurrounding a wire carrying an electric current. Magnetic flux is thenormal component of a magnetic field passing through a surface. Magneticflux lines show the direction of a magnetic field.

In embodiments of the invention and claims, a ferrimagnetic material isa substance that has groups of atoms with unequal opposing magneticmoments, resulting in a detectable magnetic field. Examples offerrimagnetic materials include magnetite (Fe₃O₄), yttrium iron garnet(YIG), cubic ferrites composed of iron oxides with other elements suchas aluminum, manganese, and zinc, and hexagonal ferrites such asPbFe₁₂O₁₉ and BaFe₁₂O₁₉ and pyrrhotite (Fe_(1−x)S). In embodiments ofthe invention and claims, ferrimagnetic materials are defined as a typeof ferromagnetic material.

The terms ferromagnetic and ferrous should not be confused. Ferrousmaterials contain iron. There are ferromagnetic materials (such ascobalt, nickel, and rare earth magnets) that are not ferrous (do notcontain iron). There are ferrous (iron-containing) materials (such asaustenitic stainless steels) that are not ferromagnetic. Not allstainless steels are austenitic. Martensitic stainless steels areferromagnetic.

Metal detectors work on a different principle than the ferromagneticdetectors described herein. Metal detectors rely on electricalconductivity of a metal. The ferromagnetic object detectors describedherein:

-   -   (a) detect ferromagnetic materials;    -   (b) do not detect metals if they are not ferromagnetic;    -   (c) do not detect ferrous materials if they are not        ferromagnetic; and    -   (d) detect non-metals and non-ferrous metals if they are        ferromagnetic.

The following table gives examples of ferrous, ferromagnetic, and metalobjects to illustrate differences in what is detected by a metaldetector and a ferromagnetic object detector based on the abovedescription of what each detector can sense:

Material Ferromagnetic Ferrous Metal Iron Yes Yes Yes 316 stainlesssteel No Yes Yes 410 stainless steel Yes Yes Yes Nickel Yes No YesTitanium No No Yes Aluminum No No Yes Rare earth magnet Yes No NoPolyethylene plastic No No No

In embodiments of the present invention and claims, passiveferromagnetic object detection means sensing a ferromagnetic object inan ambient magnetic field. Passive ferromagnetic object detectionsystems do not rely on actively creating a local magnetic field and/orlocal electromagnetic field to detect ferromagnetic objects.

In embodiments of the invention and claims, a magnetic sensor is atransducer that

measures strength, orientation, and/or a change in strength/orientationof a magnetic field. The terms magnetic sensor and magnetometer areinterchangeable in describing embodiments of the invention and claims.Magnetic sensors can operate using a variety of principles and can havea variety of outputs such as voltages, currents, and/or resistances.Examples of magnetic sensors include, but are not limited to, magneticcompasses, superconducting quantum interface devices (SQUIDs), antennas,inductive pickup coils, and fluxgate magnetometers.

In embodiments of the invention and claims, a magnetoresistance sensoris a magnetic sensor that exhibits electrical resistance in response toa magnetic field. Examples of magnetoresistance sensors includetunneling magnetoresistance (TMR) sensors, Hall effect sensors,anisotropic magnetoresistance (AMR) sensors, giant magnetoresistance(GMR) sensors, and/or magnetodiodes.

In embodiments of the invention and claims, a background electricalsignal is defined as unwanted information that can accompany atransmitted electrical signal as a result of the environment in which anelectrical signal source (such as a sensor) operates.

In embodiments of the invention and claims, electrical noise is definedas irregular

electrical fluctuations that accompany a transmitted electrical signal,but are not part of it, and tend to obscure it. In embodiments of theinvention and claims, background subtraction is the removal of abackground signal from a signal to more effectively process informationof interest. In embodiments of the invention and claims, noisecancellation is defined as the removal of noise from a signal.

2. Overview of One Embodiment of the System and Method

In one embodiment, the present invention comprises a device or methodfor locating a ferromagnetic object in a living or non-living human oranimal (i.e. organism) configured to use spatial differences in amagnetic and/or electromagnetic field at two locations to detect aferromagnetic object, wherein the first location is internal to a bodycavity and the second location is external to the body cavity. Thesystem or method:

-   -   (a) Could comprises one or more magnetic sensors at the first        location and one or more magnetic sensors at the second location        wherein the magnetic sensors used by the device could be        magnetoresistance sensors and more specifically, the        magnetoresistance sensors could be tunneling magnetoresistance        (TMR) sensors;    -   (b) Could be configured to use background subtraction and/or        noise cancellation to remove a signal responsive to sensor at        the second (external) location from a signal responsive to a        sensor at the first (internal) location;    -   (c) Could be configured for conventional open surgery and/or for        minimally invasive surgical procedures using minimally-invasive        ports;    -   (d) Could provide a detection method that is absent of        radioactivity and not dependent on a tracer detection system;    -   (e) Does not generate a magnetic field, but instead detects one        or more ferromagnetic objects based on a change in the earth's        ambient magnetic field caused by a ferromagnetic object;    -   (f) Could be configured to detect ferromagnetic objects in and        out of the surgical field of view and/or in or out of a human or        animal body cavity, both living and non-living;    -   (g) Could be configured as a wired or wireless handheld device;    -   (h) Could be configured to communicate with a remote device for        programming and/or recording information;    -   (i) Could be configured to record details of scan history        including but not limited to time, date and location;    -   (j) Could be configured to generate audible sounds, visual cues,        and/or tactile information that vary based on proximity and/or        direction to an object;    -   (k) Could be powered by a battery, a wireless power source,        and/or a wired power source;    -   (l) Could comprise an articulating tip;    -   (m) Could have adjustable sensitivity of detection; and/or    -   (n) Could comprise a display, or interface with a display, that        shows the location of any detected ferromagnetic object relative        to the location of a probe tip in three dimensions.

3. Functional Description of Embodiments of the Ferromagnetic ObjectDetector

Referring now to illustrations of the embodiments, FIG. 1A shows asurgical ferromagnetic object detector 100. This ferromagnetic objectdetector 100, and other embodiments described herein, can also detectferrimagnetic objects. The ferromagnetic object detector 100 comprises aprobe shaft 110, a probe handle 120 for holding and moving the probe,and a system controller 140. In the embodiment shown in FIG. 1A, thecontroller 140 is connected to the probe handle 120 with a cable 108.FIG. 1B shows an alternate embodiment of a surgical ferromagnetic objectdetector 102 which has a combined probe handle and system controller142. The probe shaft 110 for the alternate embodiment in FIG. 1B can befunctionally identical to the probe shaft 110 for the detector in FIG.1A. Referring to FIG. 1A and FIG. 1B, the probe shaft 110 can be 35-50cm long depending upon the application. The shaft 110 could becylindrical. It could be 1-15 mm in diameter. It could be comprised of aplastic or a surgical grade non-ferromagnetic metal covering dependingupon whether the shaft 110 is disposable or reusable. The shaft couldcomprise an internal counter that renders it expired after a pre-definednumber of uses. The shaft 110 could comprise an articulating and/ortelescoping tip located at its distal end. The shaft 110 could comprisea central flushing port for sterile processing.

Alternate embodiments of the ferromagnetic object detector can include:

-   -   (a) A system such as that shown at 100 in FIG. 1A or 102 in FIG.        1B in which all components are sterilizable and reusable;    -   (b) A system such as that shown at 100 in FIG. 1A or 102 in FIG.        1B in which the probe 110 is user removable and disposable;    -   (c) A system such as that shown at 102 in FIG. 1B in which the        entire unit is factory-sterilized, and useable for a limited        number of times before it is disposed, in order reduce the cost        and complexity of re-sterilizing combined probe handle and        system controller; and/or    -   (d) Any combination of reusable and disposable components        capable of being understood by anyone skilled in the art.

If one or more components of the ferromagnetic object detector can beused for one time, or for a limited number of times, these componentscould comprise circuitry that counts the number of times that componenthas been used and disables that component exceeds its life orcalibration period. The system could also comprise a pathogen detectoror temperature sensor that is configured to identify if that componenthas not been properly sterilized.

FIG. 1C shows the surgical ferromagnetic object detector 100 of FIG. 1Awhen used with a human patient 90 on a surgical operating table 88. InFIG. 1C, the probe shaft 110 is configured to be partially inserted intoa body cavity of the patient 90 through a surgical insertion port 70.The surgical insertion port 70 is a medical access device. The specificsurgical insertion port 70 shown in FIG. 1C comprises a trocarconfigured for placement through an abdomen during laparoscopic surgery.This same configuration could be used for other types ofminimally-invasive surgery, such as thorascopic surgery. This trocarcomprises a cannula, in the form of a hollow tube through which at leastpart of the probe shaft 110 is inserted into the abdomen of the patient90. In addition to the probe shaft 110, probe handle 120, and systemcontroller, the system in FIG. 1C also comprises a surgical trolley 86for holding the system controller 140.

FIG. 2 shows a more detailed view of the surgical ferromagnetic objectdetector of FIG. 1A, when partially inserted through the insertion port70 into a body cavity 92 of a human patient. The human body cavity 92could be any internal part of a human (or animal) patient 90. Thus, FIG.2 is similar to FIG. 1C, but shows more internal details of the bodycavity 92, probe shaft 110, and surgical insertion port 70. The probeshaft 110 comprises a probe tip magnetic sensor 202T that is located onthe distal end of the probe shaft 110. The probe tip sensor 202T isconfigured for insertion into the body cavity 92. The probe shaft 110comprises a probe base magnetic sensor 202B that is located on theproximal end of the probe shaft 110 near the probe handle 120. The probebase magnetic sensor 202B is configured for staying outside of the bodycavity 92. Information received by the probe tip magnetic sensor 202Tand probe base magnetic sensor 202B can be processed in the probe handle120 and system controller 140, as will be further described later inthis document. In the embodiment shown in FIG. 2 the probe tip magneticsensor 202T and probe base magnetic sensor 202B comprise tunnelingmagnetoresistance (TMR) sensors, described in greater detail laterherein.

FIG. 3 shows the influence of a ferromagnetic object 80 on a magneticfield vector 112 at a probe base magnetic sensor 202B and a magneticfield vector 114 at a probe tip magnetic sensor 202T that has beeninserted through a minimally invasive port 70. The dotted lines 82provide a 2-dimensional visualization of the magnetic flux lines 82 inthe region surrounding the ferromagnetic object 80. It can be seen inFIG. 3 that this ferromagnetic object 80 distorts the magnetic flux 82in its proximity. Because the ferromagnetic object 80 is located closerto the distal end of the probe shaft, the magnetic field 112 measured bythe probe tip magnetic sensor 202T is affected more than the magneticfield 114 measured by the probe base magnetic sensor 202B. Bysubtracting the magnetic probe base sensor signal 114 from the magneticprobe tip sensor signal 112, the system can be configured to detectferromagnetic objects that affect the tip sensor 202T differently thanthe base sensor 202B.

4. Magnetoresistance and Tunneling Magnetoresistance (TMR) Technology

Magnetic transducers (also referred to as magnetic sensors ormagnetometers) can be used to sense magnetic field strength and tomeasure current, position, motion, direction, and other physicalparameters. Embodiments of the inventions described herein can usehighly sensitive low-cost magnetic sensors at the probe base 202T andthe probe tip 202B in FIG. 3 . Magnetoresistance sensors are one type ofmagnetic transducer that can be used in embodiments of the invention.Examples of magnetoresistance sensors include Hall effect sensors,magneto-diodes, anisotropic magnetoresistance (AMR) sensors, giantmagnetoresistance (GMR) sensors, and tunneling magnetoresistance (TMR)sensors. The table below compares some of these sensor technologies:

Hall AMR GMR TMR Sensitivity Low Medium High Highest Temperaturestability Medium Medium Medium High Linear operating range Poor PoorPoor Good Power consumption High Medium Low Low Additional device FluxSet/reset coil None None requirements concentrator Cost High MediumMedium Low

The above table shows that TMR (tunneling magnetoresistance) sensorshave the highest sensitivity and low cost. TMR sensors can be used withbackground subtraction and noise cancellation to optimally detect aforeign ferromagnetic body in an organism without needing to generate amagnetic or electromagnetic field—i.e. the resulting device or systempassively detects changes in an ambient magnetic field caused byferromagnetic objects.

Magnetoresistance of a magnetic tunnel junction (MTJ), also known astunneling magnetoresistance (TMR), is a result of the spin-dependenttunneling effect. As shown in FIG. 4D, the typical structure of an MTJ400 is two ferromagnetic (FM) layers, 401 and 403, separated by a thininsulating (I) barrier 402, in a FM/I/FM configuration, sometimesreferred to as a “sandwich structure.” In sensor applications, oneferromagnetic layer 401 (the pinned layer) is usually designed withmagnetization that does not move in response to an applied magneticfield. The other ferromagnetic layer 403 (the free layer) is designedsuch that the magnetization moves easily in response to the appliedmagnetic field. The relative magnetization orientation of of the pinned401 and free 403 layers is representative of the strength and directionof the applied magnetic field. Because the tunneling probability forelectrons to cross the insulating barrier 402 is dependent upon therelative orientation of the free 403 and pinned 401 layermagnetizations, the resistance of an MTJ 400 indicates the appliedmagnetic field in a specific direction.

To get around the issue that electrical resistance is difficult tomeasure directly, a TMR sensor, or other type of magnetoresistancedevice can be configured into a tunneling magnetoresistance Wheatstonebridge circuit as shown at 210 in FIG. 4A. When a constant voltage isapplied between Vcc and GND, the voltage between V+ and V− will varydepending upon the resistances R1, R2, R3, and R4 based on the followingequation:

Vout=(V+)−(V−)=(Vcc)[(R4/(R3+R4))−(R2/R1+R2))]

In a typical TMR Wheatstone bridge circuit 210, R1, R2, R3, and R4 aremagnetic tunneling junctions (MTJs), with R1 and R4 having the pinnedand free layers reversed from R2 and R3 so that the resistance willincrease in R1 and R4 when the resistance in R2 and R3 decreases, andvice versa. Such a Wheatstone bridge circuit 210 with four magnetictunneling junctions (MTJs) can be fabricated on a semiconductor waferand packaged as a low-cost sensor. FIG. 4B and FIG. 4C show therelationship between Vout and magnetic field strength in Oersteds (Oe)for the TMR Wheatstone bridge circuit 210 of FIG. 4A that uses 4magnetic tunneling junctions (MTJs) and has an applied voltage (betweenVcc and ground) of 1 volt. FIG. 4B shows that a typical TMR Wheatstonebridge circuit, 210 in FIG. 4A, exhibits a linear relationship betweenmagnetic field strength (intensity) in the range of +/−5 Oersteds with aslope of about 30 millivolts/Volt/Oersted. FIG. 4C shows that thistypical TMR Wheatstone bridge circuit, 210 of FIG. 4A, exhibits anon-linear response and saturates for larger magnetic field strengths.The earth's magnetic field has an intensity on the order of 0.5Oersteds, which is the same as 0.5 Gauss or 50 microTesla. Morespecifically, the intensity of the earth's magnetic field ranges from0.3 gauss far away from the earth's magnetic poles to 0.6 gauss near theearth's magnetic poles. This means, that the typical TMR Wheatstonebridge circuit will operate in the linear region (shown in FIG. 4B) whenmeasuring ambient magnetic fields that are of the same order ofmagnitude as the earth's magnetic field (i.e. distortions of the earth'smagnetic field caused by nearby ferromagnetic objects). If Vcc is 1 voltand the distortion of the earth's magnetic field is half of the magneticfield, or a change in strength of 0.25 Oersteds at a particularlocation, a typical TMR Wheatstone bridge circuit with a linear responseof 30 mV/V/Oe would exhibit an output voltage change of 15 mV(0.25×30×2). This 15 mV peak-to-peak is a reasonable signal to beprocessed by an instrumentation amplifier or directly by an analog todigital converter.

If two TMR Wheatstone bridge circuits with identical outputcharacteristics are placed perpendicular to each other and powered bythe same excitation voltage (Vcc), one can build the equivalent of anelectronic compass that will identify the angle between the currentorientation of the electronic compass in a plane parallel to the earth'ssurface and magnetic north using the following equation:

Angle=Arctangent(Vx, Vy)

-   -   Where:        -   Angle is the angle between electronic compass and magnetic            north        -   Arctangent is an inverse tangent function that returns an            angle between −180 degrees and +180 degrees as a function of            positive or negative values of X and Y.        -   Vx is the output voltage (difference between V+ and V−) for            the Wheatstone bridge having magnetic tunneling junctions            oriented in the X-axis.        -   Vy is the output voltage (difference between V+ and V−) for            the Wheatstone bridge having magnetic tunneling junctions            oriented in the Y-axis.

Similarly, if three TMR Wheatstone bridge circuits are placedorthogonally, one can measure both the direction and the strength of amagnetic field at a particular location as a 3-dimensional vector. Onesuch 3-axis TMR sensor unit is shown at 200 in FIG. 5 . Referring to thedetails of FIG. 5 , the 3-axis TMR sensor unit 200 comprises threetunneling magnetoresistance Wheatstone bridge circuits, for theX-direction, Y-direction, and Z-direction, shown at 210X, 210Y, and210Z, respectively. The differential voltage output of each TMRWheatstone bridge circuit (210X, 210Y, and 210Z) is fed into an analogelectrical amplifier, shown at 212, and this amplified analog output isconverted to a digital binary signal using an analog to digitalconverter, shown at 214, to produce digital signals responsive to theambient magnetic field, shown at 216X for the X-direction, 216Y for theY-direction, and 216Z for the Z-direction. The analog electricalamplifiers, shown at 212 are typically semiconductor-basedinstrumentation amplifiers the use and implementation of which areunderstood by those skilled in the art.

TMR sensors, such as those shown FIG. 4A and at 210X, 210Y and 210Z inFIG. 5 are

influenced by temperature. Therefore, it is beneficial to providetemperature compensation to the circuit shown at 200 in FIG. 5 . Whenpackaging the three TMR bridge circuits into a single sensor module, itis also beneficial to provide other reference and compensation circuits,and to provide an interface to a standard digital bus architecture. FIG.6 illustrates a compensated 3-axis TMR sensor module at 202. Referringto FIG. 6 , the 3-axis TMR sensor module 202 comprises the three TMRWheatstone bridge circuits 210X, 210Y, and 210Z The low-level analogoutputs from these TMR Wheatstone bridge circuits are fed into an analogsignal multiplexer, shown a 218 and then selectively fed into the analogelectrical amplifier 212, which outputs to the analog to digitalconverter 214. Temperature compensation can be built into the sensormodule 202 by including a temperature sensor 220 that generates avoltage as a function of temperature. In the embodiment shown, theanalog amplifier 212 is responsive to the temperature sensor 220 tocompensate for temperature changes in the output signals from the TMRWheatstone bridge circuits (210X, 210Y, and 210Z) that are provided bythe multiplexer 218. An on-board magnetic source 222 can be used togenerate a reference magnetic field to calibrate the TMR Wheatstonebridge circuits (210X, 210Y, and 210Z) and magnetic sensitivity can beadjusted using a magnetic sensitivity adjustment circuit 224 that uses areference voltage.

Further referring to FIG. 6 , the output from the analog to digitalconverter 214 can be transmitted to other electronic devices over anelectronic interface 228 using an interface processor 226. The interfaceprocessor 226 could generate a signal using a protocol such as I²C, a4-wire serial protocol (commonly written as I2C), or any other protocolcapable of being understood by anyone skilled in the art. The interfaceprocesser 226 could be responsive to an oscillator 230 and the modulecould comprise a data buffer 232 to store digitized magnetic fieldstrength readings until they can be transmitted via the electronicinterface 228. The electronic interface 228 can comprise the digitalsignals shown at 216X, 216Y, and 216Z in FIG. 5 .

Summarizing the information discussed with reference to FIG. 4A, FIG.4B, and FIG. 4C, a TMR sensor when used in a Wheatstone bridge circuitwith an applied constant voltage will generate a voltage signal inresponse to the applied magnetic field. This voltage signal is typicallyin the range of millivolts. Summarizing the information presented inFIG. 5 and FIG. 6 , this voltage signal can be amplified, it can bedigitized, it can be measured in three orthogonal axes, and it can beimproved through compensation for temperature, calibration with areference magnetic field, and adjusted for sensitivity. The sensor unitshown at 200 in FIG. 5 and the sensor module shown at 202 in FIG. 6 areconfigured to be responsive to changes in magnetic field orientation andmagnetic field strength in three axes.

Referring to the magnetic field vector if the probe tip, shown at 112 inFIG. 3 , and the

magnetic field vector at the probe base, shown at 114 in FIG. 4 , if two3-TMR-sensor units, shown at 200 in FIG. 5 , are spatially separated, itis possible to measure the spatial variation of an ambient magneticfield between the location of a first 3-TMR-sensor unit and a second3-TMR-sensor unit. Thus, TMR (or some other magnetic sensor technology)can be used in embodiments of the invention to detect a spatialvariation in an ambient magnetic field caused by one or moreferromagnetic objects that differentially influence twospatially-separated 3-axis TMR sensor units.

This differential measurement of an ambient magnetic field can be moreaccurate with compensation for temperature and other factors by usingthree-axis sensor modules such as those shown at 202 in FIG. 6 are used.Referring to FIG. 3 , FIG. 6 , and FIG. 7 , two of the three-axis sensormodules shown at 202 in FIG. 6 can be used in the device of FIG. 3 . Theprobe tip sensor module has been labeled 202T in FIG. 3 , and the probebase sensor module has been labeled 202B in FIG. 3 to distinguish themfrom each other. Each of these two sensor modules, 202B and 202T, isresponsive to the projected magnetic field in the region where it islocated, as shown at 112 and 114 in FIG. 3 . This projected magneticfield can comprise (a) the earth's magnetic field, (b) electromagneticfields produced or altered by electronic equipment in the vicinity, and(c) magnetic fields generated or altered by nearly objects. Embodimentsof the invention(s) herein are configured to be responsive to magneticfields generated or altered by nearby ferromagnetic objects. Sensormodules 202B and 202T in FIG. 3 could comprise the sensor configurationshown at 200 in FIG. 5 , the sensor configuration shown at 202 in FIG. 6, or any variation on this type of sensor using any magneto-resistancedetection system or method capable of being understood by anyone skilledin the art.

Referring now specifically to FIG. 7 , the 3-axis probe tip sensormodule, previously shown in FIG. 3 , is shown at 202T and the probe basesensor module, previously shown in FIG. 3 , is shown at 202B. Thesemodules, 202T and 202B, can be similar to the sensor modules shown at202 in FIG. 6 or the sensor unit shown at 200 in FIG. 5 . The outputfrom these sensor modules, 202T and 202B, are digital signals that areresponsive to the X-axis, Y-axis, and Z-axis magnetic fields in theregions of the two sensor modules, 202T and 202B. In order to use thesedigital signals, shown at 238X, 238Y, 238Z, 239X, 239Y, and 239Z in theembodiments described herein, the two sensors must be calibrated todetermine offsets and gains, as will be described with reference to FIG.9 . This calibration process provides digital offsets and digital gainvalues to be applied to the output signals from the sensor modules,including:

-   -   The X-axis digital offset and digital gain for the tip sensor        module, shown at 240X;    -   The Y-axis digital offset and digital gain for the tip sensor        module, shown at 240Y;    -   The Z-axis digital offset and digital gain for the tip sensor        module, shown at 240Z;    -   The X-axis digital offset and digital gain for the base sensor        module, shown at 241X;    -   The Y-axis digital offset and digital gain for the base sensor        module, shown at 241Y; and    -   The Z-axis digital offset and digital gain for the base sensor        module, shown at 241Z.

Adjusting the offset and gain of the outputs from the two 3-axis sensormodules creates a set of six normalized digital magnetic field strengthsignals, which can be passed through a first digital filter, as shown at244X, 244Y, 244Z, 245X, 245Y, and 245Z in FIG. 7 . Then the differencesin these filtered digital signals can be determined as shown at 248X,248Y, and 248Z, before these difference outputs are put through a secondset of digital filters, as shown at 250X, 250Y, and 250Z. The result isthe differential magnetic field strength signals in the X, Y, and Zdirection as shown at 260X, 260Y, and 260Z.

5. Circuit and System Using TMR Technology in a Ferromagnetic ObjectDetector

FIG. 8 show a block diagram of the surgical ferromagnetic objectdetector shown at 100 in FIG. 1A and/or the alternate ferromagneticobject detector shown at 102 in FIG. 1B using the probe base sensormodule 202B and probe tip sensor module 202T shown in FIG. 3 and FIG. 7. Referring to FIG. 8 , the probe tip magnetic sensor module 202T andprobe base magnetic sensor module 202B, located in the probe shaft 110,communicate magnetic field information to a probe controller 122,located in the probe handle 120. The probe handle 120 can comprise touchbuttons and LEDs 124, that allow the user to interact with the probecontroller 122. The probe handle 120 can further comprise a forcefeedback vibration motor 126 that provides tactile information to theuser in response to the probe controller 122, such as whether aferromagnetic object has been detected. This tactile feedback cancomprise a vibration when a foreign ferromagnetic object is detected, itcan also be used to provide haptic alerts or alarms.

Further referring to FIG. 8 , the probe controller 122 communicates witha system controller central processing unit 150 in the system controller140. The controller processor (i.e., central processing unit) 150 canalso communicate with other elements located in the system controller140, such as:

-   -   (a) One or more audio amplifiers 152, which can drive one or        more speakers 154, which could be used for audio alerts or        alarms, and/or audio signals that vary in frequency and/or        intensity in response to the detection of a ferromagnetic object        and its proximity when the ferromagnetic object is detected;    -   (b) A transmission minimized digital signal (TMDS) generator 156        that connects to a digital visual interface (DVI) and/or        high-definition multimedia interface (HDMI) 158;    -   (c) A low voltage differential signaling (LVDS) generator 160        that connects to a display screen 162 (typically a liquid        crystal display) on the system controller 140;    -   (d) A touch controller 164 that works with the system controller        display screen 162 to allow tactile inputs to be made to the        system controller;    -   (e) Various system controller touch buttons and/or light        emitting diodes (LEDs) 166 at allow the user to interact with        the system controller central processing unit 150;    -   (f) A wireless digital telemetry receiver 168, such as the        Xhibit (TRADE) Telemetry Receiver

(XTR) made my Spacelabs Healthcare, that is configured for communicatingwirelessly with other devices in a surgery suite;

-   -   (g) A universal serial bus (USB) connection 170;    -   (h) A debug terminal connection 172;    -   (i) Random access memory 174;    -   (j) Flash memory 176; and/or    -   (k) A temperature sensor 178.

The wireless telemetry receiver 168 in FIG. 8 can be used to connect thesystem

controller 140 to a variety of external devices. For example, thisreceiver 168 could be used to pair the system controller to an externaldisplay screen, an external input device such as a keyboard or mouse, oran external output device such as a printer. The system controller 140could be configured to store case history data based on referencenumbers for each case. This information could be transmitted via thewireless telemetry receiver 168 to a central information managementsystem. The wireless telemetry receiver 168 could be used to connect tothe manufacturer for software updates and remote device diagnostics andtroubleshooting.

The system in FIG. 8 also comprises an electrical power storage and/orconversion module 180. The electrical power storage/conversion module180 could be part of the system controller 140 or it could be external.The electrical power/conversion module 180 could be configured toreceive electrical power from an external alternating current (AC)source 182. Alternating current (AC) could be converted to a directcurrent (DC) at a fixed voltage using a DC power supply and isolationunit 184. The direct current (DC) power supply and isolation unit 184could comprise an isolation transformer to ensure that no voltage spikesfrom the AC source 182 could reach the patient. The “safe” DC power fromthe DC power supply and isolation unit 184 can then go into a powermanagement and reset controller 186 to drive a battery charger 188 thatcharges a battery 190 and/or goes through a voltage regulator 192 topower the ferromagnetic object detector (100 in FIG. 1A or 110 in FIG.1B).

The system of FIG. 8 could be configured without an external alternatingcurrent source 182. In this case, the DC power supply and isolationmodule 184 and power management and reset controller 186 would not beneeded. The battery charger 188 would also not be needed or it could beexternal to the system. The system could then operate directly off thebattery 190 and voltage regulator 192.

The battery 190 could be any battery capable of being understood byanyone skilled in the art. In one embodiment, the battery is a lithiumbattery, and more specifically a lithium polymer battery that isrechargeable. The voltage regulator 192 could be any voltage regulatorcapable of being understood by anyone skilled in the art. In oneembodiment, the voltage regulator has a DC output voltage of 3.3 Volts.

Light signals from the probe handle LEDs 124, the system controller LEDs166, visual cues on the display screen 162 could be of varying colorsand different shapes to locate ferromagnetic objects providing proximityand direction. Audible signals from the speaker 154 could also bevarying in intensity to provide proximity to ferromagnetic objects, andgenerate alerts and alarms. The display screen 162 could show anintensity signal as well as direction and count of the ferromagneticitems. The system could have sensitivity controls. The probe shaft 110could have telescoping features and a steerable tip.

Referring to FIG. 8 in conjunction with FIG. 1B, the functions shown forthe system controller 140 and probe handle 120 in FIG. 1A could all bein the combined probe handle and system controller, shown at 142 in FIG.1B. Referring to FIG. 8 in conjunction with FIG. 1A and FIG. 1B, thepower source and receiving unit could be directly attached to the probeshaft 110 or connected via cable. For a cable-connected unit, the powersource could be a standard 110/240-volt wall outlet.

Referring to FIG. 8 , the probe shaft 110 could be a unit that is userattachable to the probe handle 120. Thus, different probe shafts 110could be attached at different times, which would allow the probe shaftto be a pre-sterilized disposable unit, separate from the rest of thesystem. User-detachability of the probe shaft would also allow differenttypes of probe shafts to be attached for different applications. Thesealternative probe shafts will be discussed later in this document.

FIG. 9 shows a method for factory calibrating the X, Y, and Z axisdigital offset and gain values that were shown at 240X, 240Y, 240Z,241X, 241Y, and 241Z in FIG. 7 . Referring to FIG. 9 , the calibrationmethod, shown at 300, comprises the following steps:

-   -   (a) Measuring probe tip and probe base sensor module readings        (the values shown at 240X, 240Y, 240Z, 241X, 241Y, and 241Z in        FIG. 7 ) at a variety of angles in a calibrated magnetic field,        as shown at 310, and visually depicted at 312;    -   (b) Comparing the probe tip and probe base sensor module        readings (shown as 238X, 238Y, 238Z, 239X, 239Y, and 239Z in        FIG. 7 ) as shown at 314, and visually depicted at 316;    -   (c) Calculating the gain and offset values needed for an        optimized least squares linear regression line in 3 axes, as        shown at 318 and visually depicted for one axis at 320; and    -   (d) Storing these gain and offset values in a non-volatile        memory as shown at 322. Note that if the sensor modules produced        analog outputs instead of digital outputs, the gain and offset        for the sensor modules could be set using resistor values.

Note that 316 in FIG. 9 depicts a 2-dimensional polar plot in which theprobe tip readings are shown as dots and the probe base readings areshown as a connected set of lines. This polar plot can be used to adjustthe gain and offsets in two axes (for example X and Y) for the twosensors. The process must be repeated in the Z-axis and would ideally beperformed in both the XZ plane and the XY plane in step 314 of theprocess shown at 300 in FIG. 9 .

Note that 310 in FIG. 9 depicts a linear regression for one probe tip inone axis. This linear regression must be performed for each of threeaxes for each of the two sensor modules for a total of six linearregressions.

Further referring to FIG. 7 in view of FIG. 9 , it should be noted thatthe signals picked up by the magneto-resistance sensor modules areinherently noisy as a result of various types of interference, includingelectromagnetic interference. By taking a large number of readings andaveraging these over a longer time period, it becomes possible todistinguish an actual difference in a magnetic field caused by aferromagnetic object in a body cavity from ambient magnetic fields.

FIG. 10 shows an example of the content of the display screen 162 thatwas referred to and discussed with reference to FIG. 8 , and is part ofthe system controller 140 shown in FIG. 1A, FIG. 1C, FIG. 2 , and FIG. 8. The two-dimensional display screen 162 can comprise a simulatedthree-dimensional perspective of the spatial relationship of the probetip and a detected ferromagnetic object as shown at 260. This simulated3D perspective 260 can be thought of as representing a cubic volume inwhich deeper objects become smaller, as shown by the convergingperspective lines on the top, bottom and sides of the simulated cubicvolume. The simulated 3D perspective 260 can comprise a probe tipposition indicator, shown at 262, and a detected ferromagnetic objectposition indicator, shown at 266.

In the embodiments shown in FIG. 10 and FIG. 11A to FIG. 11F, the probetip position indicator 262 is always in the center of the simulatedcubic volume on the left-right (horizontal) axis 270, the up-down(vertical) axis 272, and the in-out (depth) axis 274. The central planeof the cubic volume on the in-out axis is identified by a thickhorizontal line 264 on the bottom plane of the simulated 3D perspective.The probe tip position indicator 262 is always shown in the middle ofthe cubic volume above this horizontal line 264. The probe tip positionindicator 262 always stays the same size.

In the embodiments shown in FIG. 10 and FIG. 11A to FIG. 11F thehorizontal and vertical position of the detected ferromagnetic object isshown relative to the probe tip. To show relative depth, the detectedferromagnetic position indicator 266 increases and decreases in sizedepending upon the depth of the detected ferromagnetic object relativeto the depth of the probe tip. To further show depth, the ferromagneticobject position is also projected onto the base of the simulated cubicvolume, as shown at 268. Thus, the user can compare the location of theprojected ferromagnetic object position 268 to the horizontal lineshowing the central depth plane of the probe tip 264 to determinewhether the probe tip should be moved in or out. As examples:

-   -   FIG. 10 shows a detected ferromagnetic object that is further        into the body of a patient, and further to the right and further        down, than the probe tip;    -   FIG. 11A shows a detected ferromagnetic object that is        approximately at the same depth as the probe tip, further to the        right and slightly further up;    -   FIG. 11B shows a detected ferromagnetic object that is at        approximately the same depth as the probe tip and approximately        at the same location as the probe tip, but slightly further up;    -   FIG. 11C shows a detected ferromagnetic object that is slightly        further in, but otherwise aligned with the probe tip;    -   FIG. 11D shows a detected ferromagnetic object that is further        out, further to the left, and further down from the probe tip;    -   FIG. 11E shows a detected ferromagnetic object that is further        out, but otherwise almost exactly aligned with the probe tip;        and    -   FIG. 11F shows a detected ferromagnetic object that is at the        same location as the probe tip.

In an alternative embodiment of the display screen, the size of thediameter of the circle representing the detected ferromagnetic objectcould enlarge with proximity of the detected ferromagnetic object to theprobe tip. Embodiments of the display could also comprise a light meter,shown at 290, in the form of a bar graph that changes length and colorto represent proximity of the detected ferromagnetic object to the probetip.

The display screen in FIG. 10 , and FIG. 11A to FIG. 11F is shown inblack and white. It can be appreciated that embodiments of the inventioncould have color displays and that color can be used to provideinformation of the location of a detected ferromagnetic object relativeto the probe tip. For example, red could be used to indicate aferromagnetic object that is far away, green could be used to indicate adetected ferromagnetic object that is close by and intermediate rainbowcolors such as orange and/or yellow could be used for ferromagneticobjects that are at an intermediate range from the probe tip.

The display screen 162 shown in FIG. 10 can also provide surgicalprocedure information, as shown at 280. This surgical procedure data 280can comprise:

-   -   Information describing which probe shaft is attached;    -   Time and date information;    -   Surgical case information;    -   Operator information; and    -   Patient information.

The display screen 162 shown in FIG. 10 can further comprise touchscreenfunctionality and this touchscreen functionality can be used to performand get visual feedback on functionality such as:

-   -   (a) A manually entered count of the quantity of ferromagnetic        objects detected, as shown at 282;    -   (b) An adjustment of sensitivity for detecting ferromagnetic        objects, as shown at 284;    -   (c) An adjustment of the audio volume that is emitted by the        system when a ferromagnetic object is detected, as shown at 286;        and    -   (d) The ability to go to other screens to input data and/or        change configurations, as shown at 288.

6. Alternate Embodiments With Additional Magnetoresistance Sensors

FIG. 12 shows a configuration of an alternate magnetic probe shaft 111that has more than two magnetic sensors. It can be understood that thealternate probe shaft 111 in FIG. 12 attaches to the same probe handle120 that was shown in FIG. 8 and this probe handle 120 also comprisesthe same probe controller 122, touch buttons and LEDs 124, and forcefeedback motor 126. The alternate probe shaft 111 comprises the probetip magnetic sensor 202T and probe base magnetic sensor 202B that wereshown for the probe shaft 110 of FIG. 8 . The alternative probe shaft111 further comprises an additional probe base magnetic sensor, shown at202A, an additional probe tip magnetic sensor, shown at 202U, a probeshaft magnetic sensor, shown at 202S, and an additional probe shaftmagnetic sensor shown at 202R. The probe shaft magnetic sensors (202Rand 202S) are located at an intermediate point along the probe shaft,between the probe tip sensors (202T and 202U) and the probe base sensors(202A and 202B). These additional magnetic sensors (202A, 202U, 202R,and 202S) could comprise the same elements and functionality that hasbeen described for the probe tip magnetic sensor 202T and the probe basemagnetic sensor 202B. One or more of these additional magnetic sensors(202A, 202U, 202R, and 202S) could improve the capability for the systemto (a) more precisely locate a ferromagnetic object, (b) tosimultaneously discriminate between multiple ferromagnetic objects,and/or(c) to more rapidly scan a large region. A system using thealternate probe shaft 111 having three or more magnetic sensors couldtherefore also be effectively used for non-surgical applications, suchas an external scan of a patient's body prior to an MRI (magneticresonance imaging) scan, to ensure that the patient's body has noferromagnetic objects that could create harm, by for example, heating ofthe ferromagnetic object within the living organism's body.

FIG. 13A to FIG. 13B show alternative configurations of the sensorsincorporated in the medical ferromagnetic object detectors that havebeen shown and described earlier in this document. More specifically,FIG. 13A shows a medical ferromagnetic object detector 510 comprising aprobe handle 120 and a probe shaft 110 where the probe shaft 110comprises one probe tip three axis magnetoresistance module 202T and oneprobe base three axis magnetoresistance module 202B. These componentsand their functions as part of a medical ferromagnetic object detectorwere previously illustrated and/or described with reference to FIG. 1A,FIG. 1B, FIG. 1C, FIG. 2 , FIG. 6 . FIG. 7 , and FIG. 8 , as well asother previous sections of this document.

FIG. 13B shows an alternate ferromagnetic object detector 520 thatcomprises an alternative probe shaft 522. This alternate embodiment of aprobe shaft 522 comprises two probe base three-axis magnetoresistancemodules, 202B and 202A. These could specifically be identified as afirst probe base three-axis magnetoresistance module 202B, and a second(or additional) probe base three-axis magnetoresistance module 202A. Theprobe base three-axis magnetoresistance modules 202A and 202B werepreviously shown and described with respect to FIG. 12 . This alternateembodiment probe shaft 522 also comprises a linear array of probe tipthree-axis magnetoresistance modules 524, that comprises four sensors(202T, 202U, 202V, and 202W individually labeled in FIG. 14A). Thesensors in the linear probe tip three-axis magnetoresistance modulearray 524 are aligned with the axis of the alternate probe shaft 522. Byaveraging the signals from the three-axis magnetoresistance modules inthe array 524, and comparing the results of these averages in threeorthogonal axes (X, Y, and Z) with the average signal in the X, Y, and Zorthogonal axes for the two sensors at the probe base 202A and 202B, itis possible to get a more accurate signal difference to help detect andlocate a ferromagnetic object, such as the object shown at 80 in FIG.13B, that was previously shown in FIG. 3 .

The sensors in the probe tip array 524 of FIG. 13B are horizontallyseparated. Magnetic field strength decreases as the cubic power ofdistance from a magnetic object. Similarly, the distortion of a magneticfield caused by a ferromagnetic object (such as 80 in FIG. 13B)decreases by the cubic power of distance from the ferromagnetic object.Thus, in a system not affected by any other factors, the followingequations can be used to determine the distance from a tip sensor to aferromagnetic object:

E=K/d ³

d=(E/K)^((1/3))

Where:

-   -   E=the distortion in the magnetic signal caused by the        ferromagnetic object, which can also be expressed as the        difference in the magnetic signal at the tip sensor and the base        sensor (or average of multiple base sensors);    -   d=the distance between the tip sensor and the ferromagnetic        object; and    -   K=a constant based on properties of the ferromagnetic object and        the magnetic sensor.

FIG. 14A is a close-up of a ferromagnetic object proximate to region A-Aof FIG. 13B to illustrate a two-dimensional example of how the lineararray of tip sensors 524 (comprising 202T, 202U, 202V, and 202W) of thealternate embodiment ferromagnetic object detector 520 of FIG. 13B canbe used to more precisely locate the ferromagnetic object 80 using thedistance equation provided above. Also shown in FIG. 14A are the basesensors 202A and 202B in region B-B of FIG. 13B on the distal end of theprobe shaft 522. The base sensors, 202A and 202B, can be considered tobe so far away from the ferromagnetic object 80 that there is nodistortion of the ambient magnetic field caused by the ferromagneticobject. For this example, the two-dimensional angle (in degrees) of themeasured magnetic field at each of the four tip sensors and the two basesensors is as follows:

Sensor 202W 202V 202U 202T 202A 202B Measured angle 57.78 58.19 58.9159.32 57.32 57.34 Average base angle — — — — 57.33 Angular distortion0.45 0.86 1.58 1.99

To understand these multi-sensor systems, it is important to recognizethat the distortion of an ambient magnetic field caused by aferromagnetic object is a function of the following factors:

-   -   (a) The size of the object;    -   (b) The shape of the object;    -   (c) The orientation of the object; and    -   (d) The material the object is made from; and    -   (e) The inverse cube of the distance between the object and the        sensor.

In FIG. 14A and FIG. 13B all sensors are looking at the sameferromagnetic object 80, which means that size, shape, orientation, andmaterial are the same. Thus, we can use the relative differences inangular distortion of the ambient magnetic field to calculate theinverse cube of the distance from each sensor to the ferromagneticobject. For simplicity, we will normalize the angular distortions bydividing by the largest distortion so that the largest normalized valueis 1. Based on the foregoing equations, the normalized angulardistortions (E) will be equal to K/d³, which can then be converted torelative distances (kd) from each of the sensors to the ferromagneticobject as shown below. Note that this can most easily be done bydefining a new constant k, where k=K^((−1/3))). Also note that theseresulting calculated values (W, V, U, and T) below are then theindividual relative distances to each sensor, which could also bewritten as wk, vk, uk, and tk.

202W 202V 202U 202T Angular distortion 0.45  0.86  1.58  1.99 Normalized to largest value 0.226 0.432 0.794 1.000 (K/d³) Invertprevious row (d³/K) 4.422 2.314 1.259 1.000 Cube root of previous row(kd) 1.641 1.323 1.080 1.000 (W = wk) (V = vk) (U = uk) (T = tk)

Once these calculations have been made, it is possible to calculate thelocation of the ferromagnetic object 80 using trigonometry as shown inFIG. 14B. Specifically, FIG. 14B shows the same detail for region A-Athat was shown in FIG. 14A, placed onto an XY coordinate system havingan origin at the center of sensor 202T. The four probe tip sensors andtheir midpoints shown at 202W, 202V, 202U, and 202T. The detectedferromagnetic object is at a location specified by two unknown variableslabeled as lower-case x and lower-case y. If the four probe tip sensors(202W, 202V, 202U, and 202T) are equally spaced from each other on theX-axis and the spacing distance between each probe tip sensor is adistance D, then the locations of each of the probe tip sensors can beexpressed as follows and as shown in FIG. 14B:

-   -   Sensor 202T is at the origin: (0,0)    -   Sensor 202U is one increment (D) to the left of 202T, which is        at the origin: (−D,0)    -   Sensor 202V is two increments (2D) to the left of 202T, which is        at the origin: (−2D,0)    -   Sensor 202W is three increments (3D) to the left of 202T, which        is at the origin: (−3D)

Based on the X-Y coordinate system shown in FIG. 14B, the location ofthe center (x,y) of the detected ferromagnetic object 80 can becalculated relative to each of the four sensors (202W, 202V, 202U, and202T) using the Pythagorean theorem:

-   -   The hypotenuse is the distance between each sensor and the        detected object as calculated in the table above (wk, vk, uk,        and tk).    -   The distance in the X-direction is x plus the X-coordinate for        each of the sensors as given in the list above (x for 202T, x−D        for 202U, etc.).    -   The Y-distance in all cases is y.

This results in the following equations:

(wk)²=(x−3D)² +y ²

(vk)²=(x−2D)² +y ²

(uk)²=(x−D)² +y ²

(tk)² =x ² +y ²

This results in four equations representing the four sensor locations.By setting the equations up in this way, the value for the variable tfor the sensor closest to the ferromagnetic object is 1, becausenormalized all values based on this value being 1 in the table above.Thus, there are four equations and only three unknowns (x, y, and k).The extra equation can be used to improve the accuracy of the result orto provide error bounds. For the example values given, and a spacing(D=5 mm) between the sensors these equations would be:

((1.641)(k))²=(x−(3)(5))² +y ²

((1.323)(k))²=(x−(2)(5))² +y ²

((1.080)(k))²=(x−5)² +y ²

k ² =x ² +y ²

The above four equations can then be reduced to three equations with twounknowns by substituting for k². This gives the following threeequations for our example values, any pair of which can be used to solvefor x and y using numerical methods, capable of being understood byanyone skilled in the art:

((1.641)²(x ² +y ²)=(x−(3)(5))² +y ²

((1.323)²(x ² +y ²)=(x−(2)(5))² +y ²

((1.080)²(x ² +y ²))=(x−5)² +y ²

It should be noted that the concepts shown in the two-dimensionalexample shown in FIG. 13B, FIG. 14A, and FIG. 14B that uses as lineararray of probe tip sensors, can also be applied to devices that havesensors distributed in more than one dimension to find ferromagneticobjects in all three dimensions. For example, FIG. 15A shows anotheralternative ferromagnetic object detector 530 comprising anotheralternative probe shaft 532 that has a probe tip three-dimensional arrayof three-dimensional magnetoresistance modules 540 and a probe basethree-dimensional array of three- dimensional magnetoresistance modules550. The three-dimensional probe tip sensor array 540 in the embodimentshown in FIG. 13C is conceptually similar to the one-dimensional sensorarray 524 in the embodiment shown in FIG. 13B, but instead of having adistribution of three-dimensional magnetoresistance modules alignedalong one axis, the embodiment in FIG. 13C has the modules distributedin three axes to provide even further benefits in (a) redundantlyreading the angular distortion of the magnetic field, and (b) preciselylocating a ferromagnetic object. The same applies to the use of athree-dimensional array of three-dimensional magnetoresistance sensorsin the base 550 instead of the pair of three-dimensional sensors 202Aand 202B in the embodiment shown in FIG. 13B. FIG. 15B and FIG. 15Cshows enlarged views of the probe tip and probe base sensing arrays ofFIG. 15A. In this case, both arrays are 3×3×3 arrays with all sensorsequally spaced in three orthogonal axes.

FIG. 16A shows yet another alternative ferromagnetic object detector 560comprising yet another alternative probe shaft 562 that comprisesthree-dimensional magnetoresistance sensor modules distributed along thelength and width of a two-dimensional plane of the probe shaft 560. FIG.16B is a close-up view of the planar probe shaft 462 of FIG. 16A. Theplanar probe shaft of FIG. 16A and FIG. 16B comprises:

-   -   (a) A 2×2 planar array of probe base magnetic sensor modules        shown at 202A, 202B, 202C, and 202D, and collectively labeled as        570;    -   (b) A 2×2 planar array of probe tip magnetic sensor modules        shown at 202T, 202U, 202V, and 202W, and collectively labeled as        572; and    -   (c) A planar array of probe shaft magnetic sensor modules shown        at 202G, 202H, 202J, 202K, 202L, 202M, 202N, 202O, 202P, 202Q,        202R, and 202S that are grouped into three zones shown at 574,        576, and 578.

The magnetic sensor modules shown in FIG. 16A and FIG. 16B can be usedfor zone-based sensing that further improves accuracy and detectionrange by averaging the signals for each set of sensors in a zone (270,272, 274, 276, and 278) and then uses these averages to determine asensor location using the principles that were described with referenceto FIG. 14A and FIG. 14B. Such a probe shaft 462, when coupled with anappropriate averaging and distance finding system as described hereinhas the potential for being sensitive enough to prove a sensing distancethat can be used for non-invasive scanning of a body prior to an MRIscan.

In summary, the following table compares the embodiments shown in FIG.13A, FIG. 13B, FIG. 15A, and FIG. 16A.

FIG. 13A FIG. 13B FIG. 15A FIG. 16A Application Post-surgicalPost-surgical Post-surgical Pre MRI body scan example retained retainedretained for ferromagnetic ferromagnetic ferromagnetic ferromagneticobjects object detection object detection object detection and locationand location and location Improvements Not applicable RedundancyRedundancy Usable external to over FIG. 13A improves improves the body.embodiment resolution. resolution. Can scan body Spatial separationSpatial separation faster and improves ability to improves ability todetermine location accurately locate accurately locate more easilyobject object

7. Fields of Use

Examples of fields of use for embodiments of the present invention caninclude, but are not limited to:

-   -   a. Detection of retained foreign ferromagnetic objects during or        after surgery;    -   b. Location of objects with a ferromagnetic material added to        make them detectable during or after surgery;    -   c. Detection of items in a human or animal body that are not        safe for use with magnetic resonance imaging (MRI) machines; and    -   d. Detection of fragments of shrapnel, etc. that are        ferromagnetic.

Embodiments of the present invention could be used in conjunction or aspart of a device for extracting foreign objects from a patient.Embodiments of the present invention could be used in conjunction withother detection devices that use ultrasound for example, to image andhelp the foreign object detection, identification, and extractionprocess. Embodiments of the present invention could be configured and/orused for conventional open surgery as well as minimally invasivesurgical procedures in or outside the patient's body cavity for bothhuman and animal procedures.

A number of variations and modifications of the disclosed embodimentscan also be used. While the principles of the disclosure have beendescribed above in connection with specific apparatuses and methods, itis to be clearly understood that this description is made only by way ofexample and not as limitation on the scope of the disclosure.

What is claimed is:
 1. A system for displaying the location of aferromagnetic object in a living organism, wherein: the systemcomprises: a probe shaft comprising: a distal end configured forinsertion into the living organism; and a proximal end configured forremaining outside the living organism; a probe tip magnetoresistancemodule located at the distal end of the probe shaft; a probe basemagnetoresistance module located on the proximal end of the probe shaft;and a display; the probe tip magnetoresistance module comprises a probetip X-axis magnetoresistance sensor, a probe tip Y-axismagnetoresistance sensor, and a probe tip Z-axis magnetoresistancesensor wherein the probe tip X-axis magnetoresistance sensor, the probetip Y-axis magnetoresistance sensor, and the probe tip Z-axismagnetoresistance sensor are responsive to an ambient magnetic field inthree orthogonal axes at the distal end of the shaft and wherein: theprobe tip X-axis magnetoresistance sensor generates a probe tip X-axiselectrical signal in response to the ambient magnetic field at thedistal end of the shaft; the probe tip Y-axis magnetoresistance sensorgenerates a probe tip Y-axis electrical signal in response to theambient magnetic field at the distal end of the shaft; and the probe tipZ-axis magnetoresistance sensor generates a probe tip Z-axis electricalsignal in response to the ambient magnetic field at the distal end ofthe shaft; the probe base magnetoresistance module comprises a probebase X-axis magnetoresistance sensor, a probe base Y-axismagnetoresistance sensor, and a probe base Z-axis magnetoresistancesensor wherein the probe base X-axis magnetoresistance sensor, the probebase Y-axis magnetoresistance sensor, and the probe base Z-axismagnetoresistance sensor are responsive to the ambient magnetic field inthree orthogonal axes at the proximal end of the shaft and wherein: theprobe base X-axis magnetoresistance sensor generates a probe base X-axiselectrical signal in response to the ambient magnetic field at theproximal end of the shaft; the probe base Y-axis magnetoresistancesensor generates a probe base Y-axis electrical signal in response tothe ambient magnetic field at the proximal end of the shaft; and theprobe base Z-axis magnetoresistance sensor generates a probe base Z-axiselectrical signal in response to the ambient magnetic field at theproximal end of the shaft; the display is configured to show thehorizontal, vertical, and depth relationship of the location of theprobe tip to the ferromagnetic object in response to: the probe tipX-axis electrical signal; the probe base X-axis electrical signal; theprobe tip Y-axis electrical signal; the probe base Y-axis electricalsignal; the probe tip Z-axis electrical signal; and the probe baseZ-axis electrical signal.
 2. The system as recited in claim 1, wherein:the probe tip X-axis magnetoresistance sensor comprises a probe tipX-axis tunneling magnetoresistance sensor; the probe tip Y-axismagnetoresistance sensor comprises a probe tip Y-axis tunnelingmagnetoresistance sensor; the probe tip Z-axis magnetoresistance sensorcomprises a probe tip Z-axis tunneling magnetoresistance sensor; theprobe base X-axis magnetoresistance sensor comprises a probe base X-axistunneling magnetoresistance sensor; the probe base Y-axismagnetoresistance sensor comprises a probe base Y-axis tunnelingmagnetoresistance sensor; the probe base Z-axis magnetoresistance sensorcomprises a probe base Z-axis tunneling magnetoresistance sensor; andall tunneling magnetoresistance sensors comprise magnetic tunneljunctions comprising two ferromagnetic layers separated by an insulatingbarrier wherein one of the ferromagnetic layers has a magnetization thatdoes not move in response to an applied magnetic field and the otherferromagnetic layer moves in response to an applied magnetic field. 3.The system as recited in claim 2, wherein: the display comprises atwo-dimensional display; the horizontal and vertical position of thedetected ferromagnetic object is presented as a vertical and horizontaldistance between a probe tip visual marker and a ferromagnetic objectposition indicator on the two-dimensional display; and the relativedepth of the detected ferromagnetic object as compared to the locationof the probe tip is presented using a visual effect selected from agroup of: a change in size of the ferromagnetic object positionindicator on the two-dimensional display; and a projection of theferromagnetic object position indicator onto the base of a simulatedcubic volume on the two-dimensional display.
 4. The system as recited inclaim 3, wherein: the distal end of the probe shaft is configured forinsertion into an internal cavity of a human through a trocar cannulaplaced in a body wall of the human as part of a laparoscopic surgicalprocedure; the probe tip magnetoresistance module and probe basemagnetoresistance modules comprise a Wheatstone bridge circuits; theferromagnetic object comprises an unintended retained post-surgicalforeign body comprising at least one material selected from a group ofmartensitic stainless steel, nickel, and cobalt; the system isconfigured not to be responsive to at least one material selected from agroup of austenitic stainless steel, aluminum, and titanium; the systemis further configured to detect the ferromagnetic object in response toa difference between at least one pair of signals selected from a groupof: the probe tip X-axis electrical signal and the probe base X-axiselectrical signal; the probe tip Y-axis electrical signal and the probebase Y-axis electrical signal; and the probe tip Z-axis electricalsignal and the probe base Z-axis electrical signal; the system isconfigured to generate an alarm in response to the detectedferromagnetic object wherein the alarm is selected from a group of anaudible alarm, a visual alarm, and a haptic alarm; the system isconfigured for detecting the ferromagnetic object without generating amagnetic field to detect the ferromagnetic object; the system furthercomprises a force feedback vibration motor; the force feedback vibrationmotor is responsive to the detection of a ferromagnetic object; and theprobe shaft is user attachable and replaceable.
 5. The system as recitedin claim 1, wherein: gain and offset of the probe tip X-axis electricalsignal, the probe tip Y-axis electrical signal, the probe tip Z-axiselectrical signal, the probe base X-axis electrical signal, the probebase Y-axis electrical signal; and the probe base Z-axis electricalsignal are adjusted in response to digital gain and digital offsetvalues stored in a non-volatile memory in the surgical system whereinthe digital gain and the digital offset values are determined from acalibration process comprising the steps of: measuring the probe tipX-axis electrical signal, the probe tip Y-axis electrical signal, theprobe tip Z-axis electrical signal, the probe base X-axis electricalsignal, the probe base Y-axis electrical signal, and the probe baseZ-axis electrical signal at a variety of angles in a calibrated constantmagnetic field; and calculating gain and offset values for the probe tipand probe base electrical signals in response to an optimized leastsquares linear regression calculation of a relationship of the probe tipand probe base electrical signals.
 6. The system as recited in claim 1,wherein: the system comprises a second probe tip magnetoresistancemodule; the second probe tip magnetoresistance module comprises a secondprobe tip X-axis magnetoresistance sensor, a second probe tip Y-axismagnetoresistance sensor, and a second probe tip Z-axismagnetoresistance sensor wherein the second probe tip X-axismagnetoresistance sensor, the second probe tip Y-axis magnetoresistancesensor, and the second probe tip Z-axis magnetoresistance sensor areresponsive to an ambient magnetic field in three orthogonal axes at thedistal end of the shaft and wherein: the second probe tip X-axismagnetoresistance sensor generates a second probe tip X-axis electricalsignal in response to the ambient magnetic field at the distal end ofthe shaft; the second probe tip Y-axis magnetoresistance sensorgenerates a second probe tip Y-axis electrical signal in response to theambient magnetic field at the distal end of the shaft; and the secondprobe tip Z-axis magnetoresistance sensor generates a second probe tipZ-axis electrical signal in response to the ambient magnetic field atthe distal end of the shaft; and the display is configured to show thehorizontal, vertical, and depth relationship of the location of theprobe tip to the ferromagnetic object in response to: the second probetip X-axis electrical signal; the second probe base X-axis electricalsignal; the second probe tip Y-axis electrical signal; the second probebase Y-axis electrical signal; the second probe tip Z-axis electricalsignal; and the second probe base Z-axis electrical signal.
 7. Thesystem as recited in claim 1, wherein: the distal end of the probe shaftis configured for insertion into an internal cavity of a human through atrocar cannula placed in a body wall of the human as part of alaparoscopic surgical procedure.
 8. The system as recited in claim 1,wherein: the display comprises a two-dimensional display; the horizontaland vertical position of the detected ferromagnetic object is presentedas a vertical and horizontal distance between a probe tip visual markerand a ferromagnetic object position indicator on the two-dimensionaldisplay; and the relative depth of the detected ferromagnetic object ascompared to the location of the probe tip is presented using a visualeffect selected from a group of: a change in size of the ferromagneticobject position indicator on the two-dimensional display; and aprojection of the ferromagnetic object position indicator onto the baseof a simulated cubic volume on the two-dimensional display.
 9. Thesystem as recited in claim 1, wherein: the system is further configuredto detect the ferromagnetic object in response to a difference betweenat least one pair of signals selected from a group of: the probe tipX-axis electrical signal and the probe base X-axis electrical signal;the probe tip Y-axis electrical signal and the probe base Y-axiselectrical signal; and the probe tip Z-axis electrical signal and theprobe base Z-axis electrical signal.
 10. The system as recited in claim1, wherein: the system further comprises a force feedback vibrationmotor; and the force feedback vibration motor is responsive to thedetection of a ferromagnetic object.
 11. The system as recited in claim1, wherein: the probe shaft is user attachable and replaceable.
 12. Thesystem as recited in claim 1, wherein: the system is configured fordetecting the ferromagnetic object without generating a magnetic fieldto detect the ferromagnetic object; the ferromagnetic object comprisesan unintended retained post-surgical foreign body comprising at leastone material selected from a group of martensitic stainless steel,nickel, and cobalt; and the system is configured not to be responsive toat least one material selected from a group of austenitic stainlesssteel, aluminum, and titanium.
 13. The system as recited in claim 1,wherein: the display is configured to show the distance between theprobe tip and the detected ferromagnetic object as a change in size ofan object on the display.
 14. The system as recited in claim 1, wherein:the display further comprises a count of the quantity of ferromagneticobjects detected.
 15. The system as recited in claim 1, wherein: atleast one feature on the display changes color in response to the theprobe tip X-axis electrical signal; the probe base X-axis electricalsignal; the probe tip Y-axis electrical signal; the probe base Y-axiselectrical signal; the probe tip Z-axis electrical signal; and the probebase Z-axis electrical signal.
 16. A ferromagnetic object locationvisualization system, wherein: the system comprises a probe shaft and adisplay; the probe shaft comprises: a probe tip magnetoresistance modulelocated at the distal end of the probe shaft wherein the probe tipmagnetoresistance module comprises three orthogonally-oriented probe tipmagnetoresistance sensors configured for: insertion into a body cavityof a living organism; and generating three orthogonal probe tipelectrical signals in response to an ambient magnetic field in the bodycavity on said three orthogonally-oriented probe tip magnetoresistancesensors; a probe base magnetoresistance module located at the proximalend of the probe shaft wherein the probe base magnetoresistance modulecomprises three orthogonally-oriented probe base magnetoresistancesensors configured for: remaining outside the body cavity; andgenerating three orthogonal probe base electrical signals in response toan ambient magnetic field on said three orthogonally-oriented probe basemagnetoresistance sensors; and the display is configured to show thehorizontal, vertical, and depth position of the ferromagnetic objectrelative to the location of the probe tip in response to: the threeorthogonal probe tip electrical signals: and the three orthogonal probebase electrical signals.
 17. The ferromagnetic object locationvisualization system as recited in claim 16, wherein: the threeorthogonally-oriented probe tip magnetoresistance sensors and the threeorthogonally-oriented probe base magnetoresistance sensors comprisetunneling magnetoresistance sensors; and all tunneling magnetoresistancesensors comprise magnetic tunnel junctions comprising two ferromagneticlayers separated by an insulating barrier wherein one of theferromagnetic layers has a magnetization that does not move in responseto an applied magnetic field and the other ferromagnetic layer moves inresponse to an applied magnetic field.
 18. The ferromagnetic objectlocation visualization system as recited in claim 16, wherein: thesystem is further configured to detect the ferromagnetic object inresponse to a difference in the electrical signals generated by theprobe tip magnetoresistance module and the probe base magnetoresistancemodule; the system is configured to generate an alarm in response to thedetected ferromagnetic object wherein the alarm is selected from thegroup of an audible alarm, a visual alarm, and a haptic alarm; and thesystem is configured for detecting the ferromagnetic object withoutgenerating a magnetic field to detect the ferromagnetic object.
 19. Theferromagnetic object location visualization system as recited in claim16, wherein: the display comprises a two-dimensional display; thehorizontal and vertical position of the detected ferromagnetic object ispresented as a vertical and horizontal distance between a probe tipvisual marker and a ferromagnetic object position indicator on thetwo-dimensional display; and the depth position of the detectedferromagnetic object relative to the location of the probe tip ispresented: as a change in size of the ferromagnetic object positionindicator on the two-dimensional display; and as a projection of theferromagnetic object position indicator onto the base of a simulatedcubic volume on the two-dimensional display.
 20. A method forvisualizing the location of a ferromagnetic object in a body cavity of aliving organism, the method comprising the steps of: establishing aprobe shaft that comprises: a probe tip magnetoresistance modulecomprising three orthogonally-oriented magnetoresistance sensorsconfigured for: insertion into the body cavity; and generating threeorthogonal probe tip electrical signals in response to an ambientmagnetic field in three orthogonal axes in the body cavity; and a probebase magnetoresistance module comprising three orthogonally-orientedmagnetoresistance sensors configured for: remaining outside the bodycavity; and generating three orthogonal probe base electrical signals inresponse to the ambient magnetic field in three orthogonal axes outsidethe body cavity; displaying the relative location of the ferromagneticobject to the location of the probe tip magnetoresistance module on atwo-dimensional display in response to the three orthogonal probe tipelectrical signals and the three orthogonal probe base electricalsignals, wherein: the horizontal and vertical position of the detectedferromagnetic object is presented as a vertical and horizontal distancebetween a probe tip visual marker and a ferromagnetic object positionindicator on the two-dimensional display; and the relative depth of thedetected ferromagnetic object as compared to the location of the probetip is presented using a visual effect selected from the group of: achange in size of the ferromagnetic object position indicator on thetwo-dimensional display; and a projection of the ferromagnetic objectposition indicator onto the base of a simulated cubic volume on thetwo-dimensional display.